Electricity

Voltage
Energy require by each columb of charge to work $V=\frac{W}{Q}$

Circuit types

Series

  • Series circuits do not contain any wire junctions, and can be traced as one full loop.
  • Current is constant throughout the circuit; Ammeters placed in series anywhere in the circuit will read exactly the same value
  • The total resistance of the circuit is the sum of the individual resistors/things with resistance
  • If resistors are identical, voltage will be distributed equally.

Parallel

  • Parallel circuits contain wire junctions, and multiple loops. It is not possible to trace as one loop.
  • Current splits in ratio between resistors
  • Voltage does not split between loops, and is constant
  • A voltmeter always reads constant voltage, rather than changing with resistance

Power

Power
Rate of energy transfer, rate of work done

Kirchoff's first law

The sum of the currents leaving any junction is always equal to the sum of the currents that entered it, based on the conservation of energy, no charge can be lost or made in a circuit

Kirchoff's second law

The total voltage across a circuit loop is equal to the sum of the voltage drops across the devices in that loop.

Circuit symbols

Thermistor Semiconductor that varies resistance upon ambient temperature. NTC resistance drops as temp increases. PTC is same as expected
LDR Light decreases resistance
Ground Safety wire
Capacitor Two conductive plates separated by a thin strip of insulating materials. Used to store charge, to release when voltage drops
LED Allows flow of current only in one direction, & releases light
Potentiometer Three terminal variable resistor, used to change voltage if used on 2 terminals, or 3 splits the resistance in 2
Variable resistor (Rheostat) Change resistance based on input from slider, changes current

V-I Charachteristics

When plotting graphs, make the axis the most sensible choice based on the formula used. For example, for $V=IR$ the formula can be rearranged into the common $y=mx+c$ format ($I=\frac{1}{R}V$), so $I$ should be on the $y-axis$ and $V$ on the $x-axis$.

Filament lamps

Initially, as voltage increases, current does also at a consistent rate, as the temperature is low. As the voltage further increases, the rate of current increase lowers. This is because the light bulb gets hotter, & the resistance increases. This forms the following graph: [insert filament lamp VI graph]

Resistivity

\begin{align}\rho=\frac{RA}{l}\end{align} Resistance ($R$) of a material is affected by:

  • Length ($l$)
  • Area ($A$)
  • Temperature
  • Material (resistivity of material - $\rho$)

The material's ability to conduct a current is called its resistivity, and is constant for each material.

EMF and internal resistance

\begin{align}Є=I(R+r)\end{align}
\begin{align}Є=\frac{E}{Q}=\frac{energy (J)}{charge(C)}\end{align}
EMF - $Є$
Electromotive force. The amount of energy available per unit columb charge when circuit is not complete
Internal resistance
The resistance of the cell ($r$) – shown by actual resistor within a box with the cell
Pd terminal
The potential difference available in the circuit
Pd lost
The potential difference used up by the internal resistance of the cell

AC current

AC current changes polarity in a sine wave formation: [image of AC current graph]

Root mean square

The root mean square value ($V_{rms}$) is the equivilent DC voltage. To calculate it, one should use the formula: \begin{equation}V_{rms}=\frac{V_{peak\,to\,peak}}{\sqrt{2}}\end{equation}

Oscilloscopes

y-sensitivity
Voltage per vertical division.
Time base
Unit time per horizontal division.
If the time base is set to zero a single straight, vertical line is show, indicating the peak-to-peak voltage.
V-peak
Y axis peak, the highest voltage shown.
Peak to peak
Peak to trough vertically
Drift velocity
$I=nAve$ where $I$ is current, $N$ is charged particles per unit volume, $A$ is cross sectional area in $m^2$, $v$ is drift velocity and $E$ is the charge on an electron.
Current is dependent on the speed of charged particles. Drift velocity is the average speed of electrons travelling in the conductor.